1. Field of the Invention
The present invention relates to a communication system. More particularly, the present invention relates to a method and apparatus for estimating Carrier-to-Interference and Noise Ratio (CINR) in a communication system.
2. Description of the Related Art
Future-generation communication systems are under development to provide services capable of high-speed, large-data transmission and reception to Mobile Stations (MSs). An example of a future-generation communication system is an Institute of Electrical and Electronics Engineers (IEEE) 802.16 system.
The IEEE 802.16 communication system uses Orthogonal Frequency Division Multiplexing (OFDM) that offers the benefits of Inter-Symbol Interference (ISI) cancellation through a simple equalizer, robustness against noise, and high frequency use efficiency.
The IEEE 802.16 communication system adopts Adaptive Modulation and Coding (AMC) to efficiently transmit data. AMC is a transmission scheme in which an optimal Modulation and Coding Scheme (MCS) level is adaptively selected from among preset MCS levels according to a change in a channel environment and data is encoded and modulated at the selected MCS level prior to transmission.
For implementation of AMC, an MS feeds back channel status information about a radio channel to a Base Station (BS). Specifically, the MS estimates the CINR of a received signal as the channel status information and feeds back the CINR estimate to the BS.
If the BS uses a self-configurable technology, it determines optimal operation parameters based on channel status information about signals received from neighbor BSs. The self-configurable technology is a technology for automatically setting operation parameters such as a transmit power and a Frequency Assignment (FA) to efficiently transmit data. To do so, the BS estimates the CINRs of the received signals as the channel status information about the neighbor BSs and determines optimal operation parameters based on the estimated CINRs.
The CINR of a BS can be estimated using a preamble signal.
With reference to FIGS. 1 and 2, the manner in which an MS estimates a CINR will be described. FIG. 1 illustrates a conventional method for allocating a preamble signal to subcarriers in a BS.
The BS transmits a preamble signal to the MS in the first of OFDM symbols that form a downlink frame. The preamble signal is a sync signal to establish synchronization between the BS and the MS. More specifically, the BS defines a preamble allocation available area in the first OFDM symbol and maps preamble tones that form a preamble signal to part or all of the subcarriers of the preamble allocation available area according to a preset preamble allocation scheme. Then the BS eliminates the remaining subcarriers, except for the subcarriers having the preamble tones, and transmits the preamble signal to the MS.
The preamble allocation can be considered in three ways depending on the position of the preamble signal in the OFDM symbol. As illustrated in FIG. 1, preamble tones are allocated every three subcarriers, starting from subcarrier 0 in the preamble allocation available area (a segment 0 scheme 100), every three subcarriers, starting from subcarrier 1 in the preamble allocation available area (a segment 1 scheme 102), or every three subcarriers, starting from subcarrier 2 in the preamble allocation available area (a segment 2 scheme 104).
The BS transmits the preamble signal to the MS in the OFDM symbol using one of the above three preamble allocation schemes. The MS then receives the OFDM symbol and estimates the CINR of the OFDM symbol using the preamble signal. With reference to FIG. 2, a conventional operation for estimating the CINR in the MS will be described below.
Referring to FIG. 2, the MS includes an Analog-to-Digital Converter (ADC) 200, a Fast Fourier Transform (FFT) processor 202, and a CINR estimator 214. The CINR estimator 214 has a noise power estimator 204, an interference power estimator 206, a carrier power estimator 208, a preamble code generator 210, and a CINR calculator 212.
The ADC 200 converts a preamble signal received through an antenna to a digital preamble signal. The FFT processor 202 generates a digital FFT preamble signal by FFT-processing the digital preamble signal received from the ADC 200 on an OFDM symbol basis.
The noise power estimator 204 estimates a noise power value using the digital preamble signal received from the ADC 200. The preamble code generator 210 generates a preamble code with which to demodulate the digital FFT preamble signal. The carrier power estimator 208 receives the digital FFT preamble signal from the FFT processor 202 and the preamble code from the preamble code generator 210, demodulates the digital FFT preamble signal using the preamble code, and estimates a carrier power value using the demodulated digital preamble signal.
The interference power estimator 206 calculates the signal power value of the digital FFT preamble signal received from the FFT processor 202, calculates an interference power value using the signal power value, the noise power value, and the carrier power value. Herein, the interference power estimator 206 can calculate the interference power value by subtracting the carrier power value and the noise power value from the signal power value. The CINR calculator 212 calculates a CINR using the noise power value, the interference power value, and the carrier power value.
In the above-described conventional CINR estimation method, the carrier power value and the noise power value are subtracted from the signal power value. However, when the preamble signal has no interference or the magnitude of the interference is negligibly small, there is an error between the interference power value calculated by the CINR estimator 214 and a real interference power value. As a consequence, the estimated CINR is less accurate.
Moreover, the noise power is estimated using the time-domain preamble signal in the conventional CINR estimation method. If an actual CINR of the preamble signal is large in a multi-path environment, there is an error between the noise power value calculated by the CINR estimator 214 and a real noise power value. As a consequence, the estimated CINR is less accurate.